The general characteristics of the folate transport system of Lactobacillus casei have been determined previously, and the binding protein which mediates this process has been identified, isolated, and partially characterized. Considerable information has been compiled which suggests that this hydrophobic, membrane-associated binding protein serves as the carrier of folate into the cell. The principal goals of the present study are to further establish the structural properties of the binding protein and to gain insight into the mechanism by which it mediates folate uptake. Structural information will be derived at the folate-binding site, subunit formation, and protein-protein interactions involving other components of the transport system. Experiments relating to the transport mechanism will focus on the maintenance of electroneutrality during folate transport, the identity of the energy source which is responsible for the concentrative uptake of folate, and the means by which this energy source is coupled to the transport process. The results of these studies will contribute towards our general understanding of how drugs and metabolites enter cells.